Isolated lungs, perfused with Ringer's solution will be used to study the transport of water and other lipid-insoluble molecules across the blood-gas barrier in fetal, newborn and adult rabbits. The purpose of this work is to determine equivalent pore radii for both the alevolar epithelium and capillary endothelium in the lung and to document any developmental changes which occur. Equivalent pore radii will be estimated from measurements of the solute permeability coefficients for both components of the blood-gas barrier. The data we obtain should help understand the processes involved in transcapillary exchange in the immature lung, fluid secretion in the fetal lung and the rapid reabsorption of fluid from the neonatal lung.